1. Field of the Invention
The present invention relates a thin film EL (electroluminescence) device for emitting an EL in response to the application of an electric field, and more particularly to a thin film EL device wherein the emitting layer is doped with a compound of rare earth element for providing luminescent centers.
2. Description of the Prior Art
To commercially realize multicolor displays with use of thin film EL devices, it has been strongly desired to provide multicolor luminescences with a high brightness. Thin film EL devices for producing an orange luminescence with a high brightness have already been realized with an emitting layer doped with Mn for giving luminescent centers.
The present applicant has already filed a patent application for a thin film EL device for producing a bright red luminescence (U.S. patent application Ser. No. 819,217 filed on Jan. 15, 1986). As the next step, therefore, it is desired to develop a useful thin film EL device for emitting a bright luminescence of another color (e.g., green).
When the emitting layer is made of a material prepared from a II-VI compound, such as ZnS, doped with the fluoride of a rare-earth element, EL devices emitting luminescences of various colors are obtained with use of different rare-earth elements. For example, LUMOCEN devices (D. Kahng, Appl. Phys. Lett., vol. 13, pp. 210-212, 1968) have been proposed which produce green, red, blue and white luminescences when TbF.sub.3, SmF.sub.3, TmF.sub.3 and PrF.sub.3, respectively, are used as luminescent centers. Nevertheless, these devices have problems in respect of brightness, and those having a brightness sufficient for use have yet to be developed.
An emitting layer wherein the luminescent centers are provided by the fluoride of a rare-earth element is prepared by the electron beam vacuum evaporation process using sintered pellets of a mixture of ZnS with a suitable amount of the fluoride, or by the RF (radio frequency) sputtering process using a mixture of the fluoride in the form of a powder and finely divided ZnS as the target. With the emitting layer produced by such a process, the fluoride of rare-earth element (RE) serving as the luminescent centers is incorporated in the ZnS crystals usually in the form of RE.F.sub.3 molecules, and the ratio F/RE of the fluorine atoms F to the atoms of rare-earth element RE is 3 or very approximate to 3. However, the rare-earth fluoride which is in the form of a relatively large molecule, when incorporated in ZnS crystals, impairs the crystallinity of the neighboring portions of the host material, entailing a reduced luminescence brightness and lower luminescence efficiency.
If it is then possible to substitute the rare earth atom RE for the zinc atom Zn, the impairment of the crystallinity of ZnS can be diminished to a lesser extent. Nevertheless, the atom of rare-earth element is trivalent (RE.sup.3+) but zinc is divalent (Zn.sup.2+), so that if RE.sup.3+ is substituted for Zn.sup.2+, there remains a plus positive charge as an excess. The charge can be offset by providing one fluorine atom with a negative valence of one (F.sup.-1) at an interlattice position. Thus, when it is assumed that all the rare-earth atoms are ideally substituted for zinc, the ratio of the fluorine atoms to the atoms of rare-earth elements in the emitting layer, F/RE, is 1.
With thin film EL devices, therefore, the emitting layer formed is subjected to a heat treatment in order to disperse the luminescent centers uniformly through the layer and improve the crystallinity of the host material of the layer. It is desired that the heat treatment be conducted at the highest possible temperature to promote the diffusion of the elements and fully substitute atoms of the rare-earth element for atoms of the emitting layer host material. However, in the prior art in the case of thin film EL devices incorporating a rare earth fluoride as the luminescent centers, the heat treatment, if conducted, lowered the luminescence brightness of the emitting layer. Accordingly, the optimum heat-treatment temperature for giving the highest brightness is usually in the range of 400.degree. C. to 500.degree. C. (see, for example, Unexamined Japanese Patent Publication SHO 59-56390). Consequently, the heat treatment which can be conducted only at a relatively low temperature fails to fully improve the crystallinity of the emitting layer host material and permits the emitting layer to have an atom ratio F/RE of about 3, making it difficult to obtain a thin film EL device of satisfactory luminescence characteristics.